GB2047001A - Electric switch - Google Patents

Abstract

A single-pole electric switch having a longitudinally-movable striking contact 41, a longitudinally-movable rod 42 biassed by a spring 49 towards the contact 41, and a struck contact 46 pivoted at one end to a fixed bracket 50 and pivotally connected by a pin-and- slot connection 53, 46a to the rod 42. The contact 46 extends obliquely across the path of movement of the contact 41 and presents to it a contact surface 46b, along which the contact 41 slides after it strikes the contact 46 on closure of the switch, so that continued travel of the contact 41 pivots the contact 46 about the bracket 50 and moves the rod 42 against its spring 49. The switch may switch a resistor 31 into a circuit in timed relationship with the closure of a parallel connected main circuit-breaker or isolator. Adjustment of the starting position of the rod 42 by means of a nut 47 enables the contact gap and contact- engagement timing of the switch to be adjusted. <IMAGE>

Description

SPECIFICATION Electric switch This invention relates to electric switches, particularly, though not exclusively, for use in inserting a closing resistor into a high-voltage circuit prior to closure of the contacts of a main high-voltage circuit-breaker, to reduce switching over-voltages.

One of the objects of the present invention is to provide an adjustable-contact switch which for example may be operated in conjunction with a main high-voltage circuit-breaker contact for switching in a resistor circuit prior to closure of the circuit-breaker contact. Other objects are to provide a switch which is free from contact bounce or chatter, and which is such that when the switch is opened (e.g. prior to the opening of main circuit-breaker contacts), a contact gap with a high dielectric strength is quickly established.

In EHV transmission systems, the cost of providing insulation to earth and between phases is not only a major prime cost in itself, but also determines the cost of transmission lines, substation equipment, wayleaves and land charges. The level of insulation required is determined both by lightening withstand voltage and by overvoltages due to switching operations.

One of the major causes of switching overvoltages is switching onto de-energised lines or partiallyenergised lines. There are two common solutions to this problem.

Firstly, the circuit-breaker or high-speed switch may be closed when, for an unenergised line the source voltage is zero. In the case of an energised line, the circuit-breaker or high-speed switch may be closed when the voltage across the contacts is zero.

This latter solution requires accurate timing which can, however, be difficult to predict especially if shunt-reactors are connected in the high-voltage circuit, and an accuracy of one millisecond is required to keep overvoltages down to an acceptable level.

The second and more usual solution is to close into the high-voltage circuit series resistance at an instant 5-10 milliseconds before operation of the main circuit-breaker contacts, by means of an auxiliary closing switch associated with the circuitbreaker. This resistance reduces the surge phenomena which take the form of overvoltages.

The resistance must be inserted into the highvoltage line with sufficient time to allow the last reflected surge to return to the auxiliary closing switch, and thus the time required is equal to the voltage scatter between phases of the closing switch plus twice the voltage transit time to the end of the line. The optimum total ohmic value of the resistance is approximately equal to the surge impedance of the line, usually 300-400 ohm. If the resistance value is too high then the major overvoltage may occur on shorting the resistance, i.e. circuit-breaker operation. If, on the other hand, it is too low then the over-voltage occurs when the auxiliary switch closes to insert the resistance. Figure lisa graph illustrating the overvoltages predicted for various line lengths.

A modification of the single resistance insertion stage referred to above would be to provide two or more resistance insertion stages to reduce the closing overvoltages down to levels 1.8 per unit or less.

According to the invention an electric switch comprises a striking contact advanceable and retractable by a drive means and a struck contact which is angularly movable about a pivot between first and second positions and which has a bias towards the first position the contacts being relatively adjustable and the striking contact being arranged so that the switch is closed by engagement of a contact surface of the struck contact in its first position by advance of the striking contact, the striking contact after the switch has thus closed continuing to advance in sliding engagement with the contact surface while pushing the struck contact about its pivot to the second position the switch being opened by retraction of the striking contact which separates from the struck contact in or close to its second position, the contact surface in the first position of the struck contact being between a first orientation parallel to the line of advance of the striking contact and a second orientation normal to that line but being nearer to said first orientation and the contact surface in the second position being at or nearer to the second orientation.

The contact surface may be a curved or angled surface which the striking contact strikes and bears on slidingly.

Preferably the contacts are relatively adjustable by means of an adjustable stop on a rod connected to the struck contact, by means of which the gap between the contacts when the switch is open, and the timing of their initial engagement, can be.

adjusted.

In a preferred construction the stem is springbiassed to provide the bias on the struck contact.

The striking contact may be attached, directly, or indirectly through a linkage, to the main moving contact assembly of an associated switch, which may be a high-voltage interrupter assembly a switch isolator or an off-load isolator, for opening and closing movement in timed relationship therewith.

The struck contact is preferably within a metal shroud.

A switch and high voltage circuit-breaker apparatus will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a graph showing typical overvoltages encountered with single resistor insertion; Figure 2 is a schematic elevation partly in vertical section showing circuit-breaker apparatus with two alternative forms of construction shown in the respective halves of the Figure providing two different iocations for such a switch; and Figures 3 and 4 are schematic vertical sections on an enlarged scale through the switch shown as located in the left-hand half of Figure 2, showing respectively the positions of the contacts at initial closure and at the point of opening.

Figure 2 shows (for one phase of a three-phase system the other two phases being similar to the phase shown) a double tank, SF6, high voltage circuit-breaker in which there are four sets of puffer-type interrupter contacts at 10, i2, 14 and 16 in series, two sets 10, 12 being in one tank 18 and the other two sets being in the other tank 20, the tanks being filled with SF6 gas at superatmospheric.pressure. The tanks 18,20 have respective metal-clad busbar trunking at 22,24 leading to respective insulated bushing terminals 26, 28.

The tanks 18,20 are connected in sealed relationship with a common housing 30 which may be either as shown at the left-hand half of Figure 2 or alternatively shown at the right-hand half. The housing 30 has two openings aligned with respective openings at the upper ends of the tanks 18,20.

The housing 30 contains an assembly 31 of resistor components forming a single resistor which is connectable in series with the load fed by the switchgear as explained below by one switch 32, or alternatively, and as preferred, two, switches 32.

In the construction shown in the left-hand half of the figure the switch 32 (or each such switch) is located within a respective tank 18 or 20. In the right-hand half the switch, or each switch, is located within the housing 30. The actual overall dimensions of the switch 32 depend on many factors and the dimensions applicable when the switch 32 is in the tank may, as shown, be different from those applicable when the switch 32 is located in the housing 30.

Whatever the arrangement, the switch 32 is operated in response to operation of the respective interrupter assembly positioned in the respective tank below.

In each tank 18,20 the respective puffer interrupter contacts 10, 12 and 14, 16 are operable by a respective vertically movable drive assembly and the or each switch 32 has a striking contact which is movable by the respective drive assembly of the respective puffer interrupter, as next described with reference to Figures 3 and 4.

Preferably, the striking contact of the switch is directly connected to the drive assembly of the puffer interrupter.

Figures 3 and 4 show one switch 32 which has a "striking" contact 41 which is of elongate cylindrical form and which is preferably connected directly to the drive assembly (not shown) of the interrupter referred to above. The contact 41 moves linearly at the same speed as the interrupter contact assembly 10,12,14,16 under the action of an operating mechanism (not shown). A rod 42 is mounted for linear displacement within a fixed electrically conducting body 43 which is connected electrically to one end of the resistor assembly 31; the other end of the resistor 31 is connected in a similar manner to a second identical switch. The rod 42 is made of conductive material such as aluminium, or may be of an insulating material, such as glass fibre impregnated with resin.The rod 42 has an enlarged end portion 42a which is slidingly supported in closefitting guide bearings 45 in the bottom end of the body 43. The rod 42 is screw-threaded at its upper end and has an adjusting nut 47 and a lock nut 48.

The adjusting nut 47 bears against an anchorage 44a on the resistor 31. The rod 42 is biassed downwardly by means of a compression spring 49 (acting between the body 43 and a shoulder at the upper end of the rod portion 42a) to a limiting position determined by the setting of the adjusting nut 47.

Frigid lever 46 constitutes a "struck" contact of the switch and is made of a conductive material, such as copper. The contact 46 is pivotally connected near one end to a bracket 50 fixed to the body 43 and' is connected electrically to the body by a flexible copper conductor strap or braiding 51. At or near the centre of the contact 46 is an elongated slot 46a which acts as a guide for a pin 53 providing a connection between the contact 46 and the rod 42.

The contact 46 is cranked and has a contact surface 46b for sliding engagement by the hemispherical tip of the striking contact 41. The surface 46b is partly concave and is that part of the undersurface of the contact 46 slidingly transversed by the striking contact 41.

A metal shroud 54 is attached to the rod 42 by a pin 55 and is arranged to move with the rod.

Afurther metal shroud 60 is provided on the body 43 at its upper end surrounding the nuts 47 and 48 and the end of the rod 42 so as to reduce the electric field concentration in that zone in accordance with estabiished good electrical design practice.

It is required to insert the resistor 31 some 5-10 milliseconds prior to closure of the main interrupter contacts 10, 12, 14, and 16. Therefore, it is arranged that the contacts 41 and 46 engage one another some 5-10 milliseconds before the interrupter contacts close. The switch provided at the other end of the resistor 31 simultaneously closes and the resistor 31 is connected, through the circuit-breaker terminals to the line circuit (not shown). Adjustment of the gap between the contacts 41 and 46 is by means of the adjusting nut 47.

Where the contact 41 is directly connected to the drive assembly of the puffer interrupter, as is preferred, the gap between the contacts 41 and 46, and the adjustment by means of the nut 47 are the sole factors determining the necessary delay between closure of the contacts 41 and 46 and closure of the interrupter contacts 10,12, 14 and 16. The fully open position of the contact 41 is indicated at 41a in ghost outline in Figures 3 and 4 so that the gap just referred to above can be seen in Figure 3.

Where a linkage is interposed between the resistor insertion switch and the interrupter drive assembly, the speed of the striking contact 41 will probably be a proportion of or otherwise different from the speed of the interrupter drive assembly and the ratio of the speeds is then a further factor determining that delay.

Figure 3 shows the "striking" contact 41 in its position in which it first strikes the "struck" contact 46. The angle of incidence of the contact 41 on the contact surface 46b is acute and the surfaces slide on engagement, the direction of relative sliding being quite close to vertical. Such sliding contact ensures a gentle engagement. The mode of engagement is glancing. There is little or no upward impulsive force applied to the contact 46 so that the contact 46 does not tend to bounce away from the contact 41 after initial engagement. The contact 41 is connected to a relatively high mass and experiences no impulsive reaction such as to cause it to bounce away from the struck contact 46. Bounce and chatter of both contacts are therefore avoided.

In the preferred form of construction, the angle of glancing engagement of the contact 41 with the contact 46 is such that, if the motion of the location at which the contact 41 engages the surface 46b is considered to be resoived into first and second components respectively parallel to, and at right angles to, the line of advance of the contact 41, the first component is by far the major one immediately after the initial engagement.

With continued upward movement from the position shown in Figure 3 the contact 41 pushes the struck contact 46 so that progressively and smoothly the second component of motion decreases to zero orto a value close to zero when the location of engagement reaches the final position shown in Figure 4.

As the contact 41 rises from the position shown in Figure 3 the contact 46 moves about its pivot on the bracket 50, and drives the rod 42 upwardly via the pin-and-slot connection 53, 46a against the spring 49. The inside face 46b of the contact 46 engaged by the end of the contact 41, is so shaped as to provide a cam and follower action, the contact 41 sliding along the contact 46, and this together with the spring bias 9 continues to prevent contact bounce or chatter. The shroud 54 moves upwards with the rod 42 through the full extent of its travel. At that point the contacts 41 and 46 are as shown in Figure 4.

As some point in the above operating sequence, some 5-10 milliseconds after initial engagement of the contacts 41 and 46, the interrupter contacts 10, 12,14 and 16 close.

The concave part of the surface 46b of the contact 46 ensures a smooth transition to the final engaged position shown in Figure 4, in which the surface 46b of the contact 46 is at or close to horizontal orientation at the location of the engagement with the contact 41. The relative geometry of the two contacts 41 and 46 has thus been changed very considerably and in a way which is important in ensuring effective and safe separation of the contacts.

The separation occurs well inside the shroud 54, the contact 46 at separation being at or very close to its position shown in Figure 4 and being to all intents initially stationary because of the very low down ward acceleration which it experiences. The relative geometry of the separating surfaces of the contacts 41 and 46 is, or is very closely nearly, such as to give optimum rate of increase of the gap between the contacts. Thus, the risk of arcing between the contacts 41 and 46 is nil or extremely small indeed.

When the circuit-breaker is tripped to open-circuit the associated line to which it is connected interrupter drive assembly descends but before the interrupter contacts 10, 12, 14 and 16 separate the contact 41 separates from the contact 46. The mass of the rod 42, the contact 46 and the shroud 54 and the strength of the spring 49 are so chosen that the acceleration of the contact 41 is approximately 10 times that of the contact 6, the rod 42 and the shroud 54, so that the contact 41 rapidly separates from the contact 46.

At the point when the interrupter contacts 10, 12, 14 and 16 separate to break the line circuit, a substantial insulation gap will have already been established between the contact 41 on the one hand and the contact 46 and the shroud 54 on the other, so that the resistor 31 is safely isolated from the line.

Thus, a sufficiently high dielectric withstand is rapidly created to minimise arcing at the contacts 41, 46.

The mode of separation of the surfaces of the contacts 41 and 46 is such that the parts of the surfaces formerly engaged continue to be essentialiy at right angles to the line of movement of the contact 41 during the initial stage of separation so that the relative geometry of the contacts 41 and 46 is at or close to the optimum so far as suppression of arcing is concerned.

The construction described with reference to Figures 3 and 4 is generally applicable also where the switch 32 is located in the housing 30 as shown in the right-hand half of Figure 2. However, the dimensions of the shrouds 54 and 60 or of other parts of the switch such as the body 43 are not necessarily the same, as indicated generally in Figure 2.

The switch described above is such that its location is not limited by the position of the interrupter contact assemblies to a particular locality in the switchgear; nor is the switch limited to metal-clad arrangements.

The resistor assembly 31 is in practice supported along its length (e.g. mid-way) by an insulating member (not shown) or by insulating supports (not shown) near its ends.

The switch described, or two such switches in certain cases, is capable of being used to insert all of the resistance in respect of a given phase. This is in sharp distinction to commonly known practice where there is usually at least one resistor insertion switch for each set of interrupter contacts such as 10, 12, 14 or 16, mentioned above. In the circuit-breaker for the phase described above there are four sets of puffer interrupter contacts 10,12, 14 and 16, and the whole of the resistance represented by the resistor 31 is inserted by the operation of two insertion switches 32.

The number of switches 32 used depends on the voltage withstand of each resistor break. Thus, a single switch 32 could be used for 420 to 525 KV and two switches 32 for 800 KV.

The resultant advantages are: a) fewer moving parts are required: it is possible to use only one or two switches with interrupter systems having up to six breaks (i.e. six sets of interrupter contacts); b) the resistor assembly can be positioned away from the switching zone so that: (i) increased insulation integrity is achieved for each resistor break because no switching debris is produced nearby; (ii) easier access to the apparatus for maintainance is possible; (iii) the enclosures or tanks for the main interrupter contact sets and drive assemblies are of reduced diameter and simpler construction; c) the drive to the insertion switch has a lower rate of fault occurrence especially where the direct coupled in-line arrangement describedabove is used.

Although the preferred arrangement is as described, the switch or switches 32 can be used in circuit-breaker apparatus of other types and configurations (not shown). For example, in live tank circuit-breaker configurations with exposed highvoltage terminals, a switch 32 and resistor 31 as described herein, located in an insulating gas-filled enclosure, can be electrically connected in parallel with each main interrupter break, the switch being driven by a linkage connected to the main interrupter drive assembly.

In the switch arrangements shown, gravity assists the return of the struck contact and the shroud 54 to the open position after opening of the switch.

However, in other arrangements (not shown) gravity assistance may be dispensed with and a spring bias may suffice.

The switch is usable also in switch isolator or off-load isolator applications, too.

In a further modification (not shown) the contact surface 46b may be a plain rectilinear surface which in the open switch condition is inclined to the line of movement of the striking contact 41.

In Figure 4 the contact surface 46b at the point of engagement with the contact 40 is at or close to an orientation exactly normal to the line of advance of the contact 40. In a modification (not shown) the whole of the surface 46b may be in that orientation in the corresponding position of the switch. Whatever arrangement is used, the contact surface has an orientation (when the contacts separate) at or close to that orientation normal to the line of advance of the contact 40. When the contact 40 first engages the contact 46, the contact surface 46b at least where it is engaged is nearer to an orientation parallel to the line of advance of the contact 40.

Claims (16)

1. An electric switch comprising a striking contact advanceable and retractable by a drive means and a struck contact which is angularly moveable about a pivot between first and second positions and which has a bias towards the first position the contacts being relatively adjustable and the striking contact being arranged so that the switch is closed by engagement of a contact surface of the struck contact in its first position by advance of the striking contact, the striking contact after the switch has thus closed continuing to advance in sliding engagement with the contact surface while pushing the struck contact about its pivot to the second position the switch being opened by retraction of the striking contact which separates from the struck contact in or close to its second position, the contact surface in the first position of the struck contact being between a first orientation parallel to the line of advance of the striking contact and a second orientation normal to that line but being nearer to said first orientation and the contact surface in the second position being at or nearer to the second orientation.

2. A switch according to Claim 1, in which the contact surface is partly concave.

3. A switch according to Claim 1 or Claim 2, in which the struck contact is within a metal shroud which is movable in response to movement of the struck contact.

4. A switch according to any preceding claim, in which the contacts are relatively adjustable by means of an adjustable stop on a rod connected to the struck contact, by means of which the gap between the contacts when the switch is open, and the timing of their initial engagement, can be adjusted.

5. A switch according to Claim 4, as dependent on Claim 3, in which the shroud is mounted on the rod.

6. A switch according to Claim 4 or Claim 5, in which the rod is connected by a pin-and-slot connection to the struck contact.

7. A switch according to Claim 4,5 or 6, in which the rod extends through a body the struck contact being pivotably mounted on the body at one end thereof and the stop being engageable with a stop surface on the body at the other end thereof or on a member secured thereto, and in which the bias on the struck contact is provided by a spring housed within the body.

8. A switch according to any preceding claim, forming part of a high voltage circuit-breaker apparatus, in which the drive means is the drive means by which main interrupter contact means of apparatus is operable and the switch is arranged to switch in and out closing resistor means.

9. A switch according to Claim 8, in which the interrupter contact means and the switch are contained in a common enclosure.

10. A switch according to any claim of Claims 1 to 7, forming part of high voltage circuit-breaker apparatus having two enclosures each containing main interrupter contact means and in which the drive means is the drive means by which the interrupter contact means are operable, the switch being in one of the enclosures and a similar switch being in the other of the enclosures, the switches being arranged to switch in and out closing resistor means contained in a housing having two openings which are aligned with respective openings of the two enclosures, the housing being secured to the enclosures in sealed relationship therewith.

11. A switch according to any claim of Claims 8 to 10 in which the or each striking contact is directly connected to the respective drive assembly of the interrupter contact means of the circuit breaker.

12. Aswitch according to Claim 11, in which the or each striking contact is of elongate form and is movable length-wise vertically by the drive means, the struck contact being angularly movable about a horizontal axis.

13. A switch according to Claim 12, in which the upper end of the striking contact is hemispherical.

14. A switch substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.

15. A switch forming part of high voltage circuit breaker apparatus substantially as hereinbefore described with reference to Figure 2, left-hand modification, and Figures 3 and 4.

16. A switch forming part of high voltage circuit breaker apparatus substantially as hereinbefore described with reference to Figure 2, right-hand modification, and Figures 3 and 4.